The geodynamic history of a region is archived in its geologic record which, in turn, may reflect deformation patterns that causally can be related to certain configurations of paleostresses. In the Oslo Region, the exposed geological record ranges from Precambrian high-grade metamorphic rocks through Cambro-Silurian sedimentary rocks to Permo-Carboniferous sedimentary and magmatic rocks, the latter being related to the development of the Oslo rift system. We investigate the kinematics of outcrop-scale faults to derive the diversity of paleostress states responsible for the observed strain. For this purpose, we combine different graphical and numerical approaches to separate heterogeneous fault-slip data sets and estimate the associated reduced stress tensors. A reduced stress tensor consists of the directions of the three principal stress axes with σ1 ≥ σ2 ≥ σ3 and the ratio of principal stress differences, R = (σ2 − σ3)/(σ1 − σ3). The present study gives evidence for three major regional paleostress fields that affected the Oslo Region: A compressional stress field with a NW–SE-directed maximum compression (σ1) has been identified as a Caledonian imprint. The most prominent regional stress field, however, is tensional, characterised by a WNW–ESE directed minimum compression (σ3), and related to the Permo-Carboniferous rifting. Later, the area was affected by a wrench regime with a roughly N–S directed σ1 and a maximum age of Permian, whereas the absolute timing of this stress field is unclear due to the lack of exposed rocks younger than Permian. For a large number of estimated stress states, none of the principal axes are sub-vertical. These “oblique” paleostresses cannot be integrated into a common regional stress field but rather correspond to local effects of intense magmatic activity during the Permo-Carboniferous phase of rifting. The present study tends to show that the Oslo Region remained unaffected by major tectonic activity for much of the Mesozoic and Cenozoic – which is when the Central European Basin System to the south experienced several phases of intense deformation.

The Oslo Rift is the northernmost part of the Rotliegendes basin system in Europe. The rift was formed by lithospheric stretching north of the Tornquist fault system and is related tectonically and in time to the last phase of the Variscan orogeny. The main graben forming period in the Oslo Region began in Late Carboniferous, culminating some 20–30 Ma later with extensive volcanism and rifting, and later with uplift and emplacement of major batholiths. It ended with a final termination of intrusions in the Early Triassic, some 65 Ma after the tectonic and magmatic onset. We divide the geological development of the rift into six stages. Sediments, even with marine incursions occur exclusively during the forerunner to rifting. The magmatic products in the Oslo Rift vary in composition and are unevenly distributed through the six stages along the length of the structure.

The presence or absence of a thermally anomalous mantle plume during the formation of the widespread Carboniferous–Permian magmatism of northern Europe is examined. The geochemistry of representative samples from the extensive Carboniferous–Permian dyke and sill intrusions across northern Europe are reported in order to ascertain whether they have a common ‘plume’ source. Both tholeiitic and alkaline magmas have diverse trace element compositions. Alkaline samples with relatively low Ti and Nb/La < 1 are considered to originate in the lithospheric mantle and those with Nb/La > 1 from the asthenosphere. The tholeiites have a close affinity to E-MORB but have mixed with variable amounts of lithosphere and upper crust. Tectonic reorganisation and decompression melting of a trace element-enriched mantle is considered to have controlled the Carboniferous–Permian magmatism, which contains no coherent geochemical evidence for a single plume-related thermo-chemical anomaly.

This study focuses on Late Carboniferous - Permian tectonics and related magmatic activity in north-western Europe, and specifically in the Skagerrak, Kattegat and North Sea areas. Special attention is paid to the distribution of intrusives and extrusives in relation to rift/wrench geometries. A large database consisting of seismic and well data has been assembled and analysed to constrain these objectives. The continuation of the Oslo Graben into the Skagerrak has been a starting point for this regional study. Rift structures (with characteristic half-graben geometries) and the distribution of magmatic rocks (intrusives and extrusives) were mapped using integrated analyses of seismic and potential field data. For the analysis of the Sorgenfrei-Tornquist Zone and the North Sea seismic and well data were used. The rift structures in the Skagerrak can be linked with extensional structures in the Sorgenfrei-Tornquist Zone in which similar fault geometries have been observed. Both in the Skagerrak and the Kattegat, lava sequences were deposited which generally parallel the underlying Lower Palaeozoic strata. This volcanic episode therefore, predates main fault movements and the development of half-grabens filled with Permian volcaniclastic material. Upper Carboniferous - Lower Permian extrusives and intrusives have also been found in wells in the Kattegat, Jutland and the North Sea (Horn and Central grabens). Especially in the latter area, the dense seismic and well coverage has allowed us to map out similar Upper Palaeozoic geometries, although the presence of salt often conceals the seismic image of the underlying strata and structures. From the results, it is assumed that the pre-Jurassic structures below large parts of the Norwegian-Danish Basin and northwards into the Stord Basin on the Horda Platform belong to the same tectonic system.

The Carboniferous-Permian evolution of north-western Europe has recently been the focus of an EC funded Training and Mobility of Researchers (TMR) project “Carboniferous-Permian Rifting in Europe”. One of the main goals of this project was to produce a new map for this time period showing the distribution of Late Carboniferous – Early Permian (Lower Rotliegend) volcanics, dykes and sills and the extent of the tectonic structures of the Early – Late Permian (Upper Rotliegend) sedimentary basins (better known as the Southern and Northern Permian Basins). In order to produce this map, an overview of all the available literature was made. The new map was completed based on our own interpretations from seismic and borehole data. Unpublished data were available through industrial partners associated with the PCR-project.

The Permo-Carboniferous evolution of the central North Sea is characterized by three main geological events: (1) the development of the West European Carboniferous Basin; (2) a period of basaltic volcanism during the Lower Rotliegend (latest Carboniferous - early Permian); and (3) the development of the Northern and Southern Permian Basins in late Permian times. The timing of the late Carboniferous - Permian basaltic volcanism in the North Sea is poorly constrained, as is the timing of extensional tectonic activity following main inversion during the Westphalian due to the propagation of the Variscan deformation front. Results of high precision Ar-Ar dating on basalt samples taken from a core from exploration well 39/2-4 (Amerada Hess) in the UK sector of the central North Sea, suggests that basaltic volcanism was active in the late Carboniferous, at c. 299 Ma. The presence of volcanics below the dated horizon, suggests that the onset of Permo-Carboniferous volcanism in the central North Sea commenced earlier, probably at c. 310 Ma (Westphalian C). This is contemporaneous with other observations of tholeiitic volcanism in other parts of NW Europe, including the Oslo Graben, the north-east German Basin, southern Sweden and Scotland. Interpretations of available seismic data show that main extensional faulting occurred after the volcanic activity, but the exact age of the fault activity is difficult to constrain with the data available.

The deep seismic sounding DSS profiles BALTIC, including its southern continuation, the Sovetsk Kohtla Jarve (SKJ) profile, SVEKA, the northern part of BABEL, POLAR, FENNIA and Pechenga-Kovdor-Kostomuksha, were used in studying the present-day thermomechanical structure of the central Fennoscandian Shield. These profiles are located in different tectonic units, which represent different stages in Precambrian crustal and lithospheric growth. First, present-day geotherms were constructed for several points along the DSS profiles. Successively, strength envelopes were calculated using the obtained geotherms and rheological flow laws. Variations in strain rate were also considered in the computations of the strength envelopes. The integrated crustal and lithospheric strengths, the thicknesses of the mechanically strong crust (MSC) and mechanically strong lithosphere (MSL), and the rheological thickness of the lithosphere were derived from these strength envelopes. The obtained mechanical structures for different regions were analysed and compared with other geophysical data; e.g., seismicity-depth and isotherm-depth distributions. The rheological results show lateral variations in the lithospheric strength reflecting the geometry of the lithosphere and following roughly the same trend as the geochronological development of the Fennoscandian Shield. The mechanical structure shows distinct decoupling of the weak lower crust and the strong upper mantle, particularly with a wet rheology. This decoupling interrupts the transmission of the differential stress from the brittle upper crust to the ductile lower crust and through it to the mantle lithosphere. The weak lower crustal layer is also detected with a dry rheology in the Svecofennian area, whereas in the Archaean side, it is not distinct. The assumed frictional transition temperature of 350 C varies between the depths of 25 and 44 km with an average value of 35 km. This is in good agreement with the observed focal depth limit of 31 km. Consequently, it seems that the velocity weakening/velocity strengthening explains best the real lower boundary of seismicity.

The three intracratonic sedimentary basins located in central Baltoscandinavia, namely the Bothnian Gulf basin, the Bothnian Sea basin and the Baltic basin, developed in response to Middle Proterozoic and Late Proterozoic tectonic events, separated in time by about 800 Ma. Only the Baltic basin was subsequently affected by Caledonian orogenesis and Mesozoic rifting. Crustal extension was minor or did not take place during the Proterozoic basin evolution phases. However, according to the Moho topography, crustal thinning did take place. This was probably a result of subcrustal magmatism. On a craton-wide scale, the ages of granitoids, which intruded during the Middle Proterozoic basin formation, generally decrease from east to west. This fact, combined with the evidence provided by mantle-derived flood basalt magmatism, points to a moving asthenospheric diapir as the cause for basin development. Asthenospheric upwelling was probably also responsible for the second, Late Proterozoic, basin evolution phase, as evidenced by the lack of crustal thinning and extension, and the occurrence of tholeiitic intrusions. In addition, a Late Proterozoic thermally induced palaeo-high, located at about the position of the intracratonic basins, is compatible with indications from glaciations. As the ages of Late Proterozoic intracratonic basins also decrease from east to west across the craton, the location of asthenospheric diapirism during this time interval was also moving. For the Fennoscandian lithosphere, the presence of fundamental lithospheric weakness zones (e.g. terrane boundaries) might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the Fennoscandian Shield. The spacing and size of the Proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated. Therefore, sublithospheric convective processes might be the cause for the asthenospheric upwellings. Such processes are related to Rayleigh--Taylor instabilities in the sublithospheric mantle. Emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere. Modelling results demonstrate that erosion of the surficial high, succeeded by cooling of the lithosphere, can explain the accumulation of early Palaeozoic sediments in the Bothnian Sea basin, taking into account post-Ordovician vertical and lateral erosion of the basin fill

Interpretation of deep seismic reflection data across the Gascoyne Margin reveals six distinct seismic facies units related to the tectono-magmatic breakup history. On the outer Exmouth Plateau four large scale units are identified: (1) an extensively block-faulted upper crust; (2) a middle-crustal unit of discontinuous, undulatory reflectors; (3) a reflection-free deep crustal unit; and (4) a lower-crustal band of low-frequency, high-amplitude reflectors. Two additional units are found near the continent-ocean boundary (COB); (5) seaward-dipping reflectors (SDR); and (6) landward-dipping reflectors in the lower crust below the SDR. The lower-crustal high-reflectivity band, located near the top of a high-velocity unit (Vp > 7 kms 1), is interpreted as magmatic underplating. There is a spatial correlation between the underplated area and the presence of extensive upper-crustal block-faulting and intrusive rocks in the shallow crust. The undulatory middle-crustal reflector unit is also only identified in the outer plateau area, and is interpreted as a zone in which the upper-crustal faults terminate. The inner parts of the margin consist of a deep basin showing little upper-crustal faulting and no evidence of middle crustal deformation or underplating. Theoretical modeling of the effect of rifting and magmatic underplating on crustal strength profiles suggests that the brittle-ductile transition may migrate at least 5 km upwards during several million years after the underplating event. Based on the seismic interpretation and crustal strength modeling we propose that the seismic structure of the outer Exmouth Plateau is severely modified by a transient change in the crustal rheological structure associated with magmatic underplating.

The post-Svecofennian tectonic development of southern Finland is controlled by intrusion of rapakivi granites (and associated rocks), reactivation of Svecofennian wrench zones, formation of sedimentary basins and successive intrusion of olivine dolerite dykes and sills. Relative age determinations have revealed that fault reactivation acted before, simultaneously and after intrusion of the rapakivi granites. Results of 40Ar/39Ar geochronometry of the Porkkala-Mäntsälä fault (30 km west of Helsinki) reveal ages predominantly in the range 950-1300 Myr. These ages are all significantly younger than the intrusion age of the rapakivi granites. It is suggested that these ages represent tectonic events related to the intrusion of olivine dolerite dykes and sills in SW Finland and the Sveconorwegian Orogeny active further west. 40Ar/39Ar ages of a sample taken from the Obbnäs granite (U-Pb zircon ages of 1645 ± 5 Myr) show ages predom-inantly in the range of 1400-1550 Myr. These ages are suggested to represent either cooling ages of the granite or ages associated with the formation of the sedimentary grabens.

Heeremans, Michel & Faleide, Jan Inge (2008). The Tectono-Magmatic Evolution of the Skagerrak Graben: new Insights, old Ideas.
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The offshore Skagerrak Graben, which is defined as the southwards continuation of the onshore Oslo Graben, has recently been the focus of some debate regarding its tectonomagmatic development. Two hypotheses for its development exist i.e. 1) a magmatically active symmetrical rift zone with significant magmatic underplating, and 2) an a-magmatic, asymmetrical rift controlled by pre-existing Precambrian low- angle shear zones. The onshore Oslo Graben to the north is a well known continental rift system with opposing half grabens and excessive magmatism that developed during late-Carboniferous-Permian times. Seismic and well data from the Sorgenfrei-Tormquist Zone to the south have shown similar rift geometries as found in the Oslo Graben as well as in the Skagerrak Graben and also show the presence of magmatic products of late-Carboniferous- early Permian age. The spatial relationship between the Oslo Graben, Skagerrak Graben and Sorgenfrei- Tornquist Zone also suggest a genetic link. Integrating both geological (structural geological, petrological) and geophysical (potential field data, seismic data) data, we will summarize some important observations regarding the geometry, vertical movements and magmatism of the graben in both time and space. We will put focus on: - Magmatism in the Oslo Graben is much more pronounced than predicted by stretching models (decompressional melting). This suggest increased mantle potential temperature and an upward shift of the solidus - A regional E-W stress field gives rise to oblique components in the Skagerrak Graben (sinistral) and the Sorgenfrei-Tornquist Zone (dextral). Any effects of structural inheritance? - There are large differential vertical movements within the rift i.e. a thick pre-rift (Lower Paleozoic succession) is down-faulted and preserved within the main graben segments, the same unit is removed by rapid erosion within other parts of the rift. This would suggest a maximum uplift and erosion at the inner corner between the Skagerrak Graben and the Sorgenfrei-Tornquist Zone. Is there any relationship to underplating and magmatism? - What was the width of the rift system compared to the main graben segments? - Is there ny syn-rift uplift? Why did not the Zechstein Sea transgress the Oslo Rift? - What caused the apparent lack of post-rift subsidence in Early-Middle Triassic times? There is a clear regional unconformity which has been transgressed and buried in Middle?-Late Triassic time All observations point towards active magmatic activity during the creation of the Skagerrak Graben with magmatic accretion below and/or in the lower crust.

Permo-Carboniferous dykes and sills of various composition are encountered in southern Norway, Scotland and southern Sweden. They most likely belong to the large tectono-magmatic episode which was also responsible for the formation of the Oslo Graben and the Northern German Basin. Dykes and sills are regarded as reliable stress indicators and a detailed analyses of the Permo-Carboniferous dykes and sills would provide us with valuable structural constraints about this prominent tectono-magmatic episode in NW Europe. In order to do this a large database on dykes and sills of Permo-Carboniferous age has been put together including data on structure, petrology, geochemistry and geochronology. Data documented and published over the last hundred years has been supplemented with new data gathered over the last years. Connecting high precision Ar/Ar dating, chemical and isotope variations and detailed structural data of various dykes and sills gives us a powerful tool to examine the structural evolution of the area over time in relation to the magmatic source.

The Central European Basin System (CEBS) reveals a complex structure resulting from a polyphase deformation history since the Permian. The basin system is framed by two major NW-striking fault systems, the Elbe Fault System (EFS) in the south and the Tornquist Zone (TZ) in the north. We investigate the kinematics of faults on the outcrop scale to estimate the diversity of paleostress states responsible for the observed strain. The method used to estimate the reduced stress tensors for the measured fault populations integrates graphical and numerical approaches of fault-slip analysis. This technique facilitates the separation of heterogeneous data sets and guarantees each estimated stress state to fulfil both the criterion of low misfit angles and the criterion of high shear-to-normal stress ratios. For the basin-wide reconstruction of paleostress fields, the orientations of more than 850 faults with known slip directions have been sampled from outcrops across the EFS, where Upper Carboniferous, Upper Permian, Middle Triassic, Upper Jurassic, and Upper Cretaceous rocks are exposed. In addition, more than 4600 fault-slip data from the Oslo Graben area north of the TZ have been sampled from rocks of Precambrian to Permian ages. For both study areas, a polyphase paleostress history is established. The most prominent paleostress field reconstructed for the EFS is characterised by a horizontal N-S- to NE-SW-directed maximum compression combined with a relatively low stress ratio. This stress field can clearly be assigned to a phase of basin inversion which is known to have affected the entire CEBS in the Late Cretaceous-Early Tertiary. The signs of earlier phases of deformation are widely overprinted in the study area. On the contrary, the most prominent paleostress field reconstructed for the Oslo Graben area corresponds to radial tension and is related to the phase of rifting and graben formation during the Late Carboniferous and Early Permian. The distribution of preserved traces of deformation indicates that the crustal domains south of the TZ have intensely been affected by late Mesozoic and Cenozoic stress fields as opposed to areas north of the TZ where the effects of Paleozoic and early Mesozoic stress fields are preserved.

The CEBS is framed by two NW-SE-oriented fault systems, the Tornquist Zone in the north and the Elbe Fault System in the south. Along the Elbe Fault System rocks of Late Carboniferous, Middle Triassic, Late Jurassic, and Late Cretaceous contain striated faults that have been investigated. The Tornquist Zone runs south of the Oslo Graben in Southern Norway, where Permian rocks bear the traces of paleo stress fields that have affected the area since the Permian. Though rocks of different ages are exposed in consequence of inversion, the two areas have been affected by the same regional stress fields since the Permian. We analyse the paleostress fields that can be derived from fault slip data and evaluate how the two areas compare. We perform a fault-slip analysis based on a large number of mesoscale striated faults sampled along the inverted southern and northern margins of the CEBS. For each fault-slip pattern sampled we derive the corresponding stress state(s) in terms of the reduced stress tensor consisting of (1) the orientations of the three principal stress axes σ1, σ2 and σ3 with σ1¡Ýσ2¡Ýσ3 and (2) the ratio of principal stress differences, R=(σ2−σ3)/(σ1−σ3) with 0¡ÜR¡Ü1. Despite the different ages of investigated rocks (Late Carboniferous, Permian, Middle Triassic, Late Jurassic, and Late Cretaceous), we find amazingly consistent stress configurations throughout the entire area which can mainly be combined to a reduced number of stress fields. We present the distribution and local variations of the detected stress states, discuss possible constraints on their relative chronology and address some questions that arise from our results.

In order to discuss the development of the Oslo Graben in terms of plate vs. plume models, it is most important to understand its spatial and temporal relationship with other areas in NW Europe. Despite its “abnormal” development, the Oslo Graben is temporally associated with nearby magmatic events in areas such as southern Sweden, north-west Germany, Scotland and the central North Sea (see, for example, Wilson et al. (2004) for an overview of Carboniferous-Permian magmatism and rifting in north-western Europe). The most important of these events is the emplacement of a suite of alkaline and tholeiitic basalts at ca. 300-290 Ma (the Carboniferous-Permian boundary) (Sundvoll et al., 1990; Breitkreuz & Kennedy, 1999; Heeremans et al., 2004b; Monaghan & Pringle, 2004; Timmerman, 2004). Except for this basaltic spike, all the areas show different tectonomagmatic developments. The focus of this web page will be on the formation of the Oslo Graben. This graben has long been known as a highly-magmatic continental rift, with accommodation and transfer zones separating the different graben segments (Olaussen et al., 1994). It has been studied for over a hundred years and a large amount of geological and geophysical data have been acquired. Despite this, scientists still argue about its origin. The present paper will try to shed some light on the discussion regarding whether a plume contributed to the origin of the Oslo Graben or not. Before starting the discussion, I give a short introduction to the tectonomagmatic development of the Oslo Graben.

During the Late Carboniferous and Early Permian an extensive magmatic province developed within northern Europe, intimately associated with extensional tectonics, in an area stretching from southern Scandinavia, through the North Sea, into northern Germany. Within this area magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania in southern Sweden, the island of Bornholm, the North Sea and northern Germany. Available geochemical (major- and trace-element, and Sr-Nd isotope, data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. Peak magmatic activity was concentrated in a narrow time-span from c. 300 to 280 Ma. The magmatic provinces developed within a collage of basement terranes of different ages and lithospheric characteristics (including thicknesses), brought together during the preceding Variscan orogeny. This suggests that the magmatism in this area may represent the local expression of a common tectono-magmatic event with a common causal mechanism. Available geochemical (major and trace element and Sr-Nd isotope data) and geophysical data are reviewed to provide a basis for understanding the geodynamic setting of the magmatism in these areas. The magmatism covers a wide range in rock types both on a regional and a local scale (from highly alkaline to tholeiitic basalts, to trachytes and rhyolites). The most intensive magmatism took place in the Oslo Graben (ca. 120000km 3 ) and in the NE German Basin (ca. 48000km 3 ). In both these areas a large proportion of the magmatic rocks are highly evolved (trachytes-rhyolites). The dominant mantle source componet for the mildly alkali basalts to subalkaline magmatism in the Oslo Graben and Scania (probably also Bornholm and the North Sea) is geochemically similar to the Prevalent Mantle (PREMA) component. Rifting and magmatism in the area is likely to be due to local decompression and thinning of highly asymmetric lithosphere in responses to regional stretching north of the Variscan Front, implying that the PREMA source is located in the lithospheric mantle. However, as PREMA sources are widely accepted to be plume-related, the possibility of a plume located beneath the area cannot be disregarded. Locally, there is also evidence of other sources. The oldest, highly alkaline basaltic lavas in the southernmost part of the Oslo Graben show HIMU trace element affinity, and initial Sr-Nd isotopic compositions different from that of the PREMA-type magmatism, These magmas are interpreted as the results of partial melting of enriched, metasomatised domains within the mantle lithosphere beneath the southern Olso Graben; this source enrichment can be linked to migration of carbonatite magmas in the earliest Paleozoic (ca. 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems have provided an important magma source components.

During late Carboniferous and early Permian time an extensive magmatic province developed within northern Europe. Magmatism was unevenly distributed, concentrated mainly in the Oslo Graben and its offshore continuation in the Skagerrak, Scania, the Baltic Sea, Bornholm, the North Sea, northeast England, and northern Germany. Peak magmatic activity was concentrated in the narrow time span 300-280 Ma. The various magmatic provinces developed basement terranes of different ages and lithospheric characteristics. This suggests that the magmatism in different parts of northern Europe may represent the local expressions of a common tectono-magmatic event with a common causal mechanism. The dominant mantle source component for the most primitive mafic magmas generated within the province (ranging from basanites and alkali basalts to subalkaline tholeiites) is sub-lithospheric; it is geochemically similar to the PREMA component which is widely accepted to be plume-related. Locally, however, distinctive alkaline magma types were generated by partial melting of enriched, metasomatised domains within the mantle lithosphere; in the Oslo Graben this source enrichment can be linked to intrusions by carbonatite magmas in the earliest Paleozoic (ca 580 Ma). Within northern Germany, mantle lithosphere modified by subduction-related fluids from Variscan subduction systems may provide an important magma source component.

Permo-Carboniferous dykes and sills of various composition are encountered in southern Norway, Scotland (Whin-Midland Valley) and southern Sweden (Scania). They most likely belong to the large tectono-magmatic episode which was also responsible for the formation of the Oslo Graben and the Northern German Basin. It has been suggested that the trends of the dykes indicate that they are part of a radiating dyke swarm probably connected to a mantle plume (e.g. Ernst and Buchan, 1997). Dykes and sills are regarded as relaible stress indicators and a detailed analyses of the Permo-Carboniferous dykes and sills would provide us with valuable structural constraints about this prominent tectono-magmatic episode in NW Europe. In order to do this a large database on dykes and sills of Permo-Carboniferous age has been put together including data on structure, petrology, geochemistry and geochronology. Data documented and published over the last hundred years has been supplemented with new data gathered over the last 3 years by the PCR Network. Systematic sampling for geochemical and geochronological analyses have been carried out in Norway, Sweden and Scotland. Connecting precise 40Ar/39Ar radiometric dating, chemical and isotope variations and detailed structural data of various dykes and sills gives us a powerful tool to examnine the structural evolution of the area over time in relation to the magmatic source.

From May 21-27, 1999, a wide-angle seismic experiment was carried out in the Skagerrak Graben. The experiment was a combined effort of the department of Geology, University of Oslo and Geomar, Kiel (as partners in the EU-TMR project: Permo-Carboniferous Rifting in Europe (PCR) - Magmatism, Geodynamics and Thermal Evolution of the Lithosphere), the Geological Institute, University of Copenhagen (Denmark) and the Geophysical Institute, University of Hamburg (Germany). The involvement of the University of Hamburg made it possible that the off-shore part of the experiment could be carried out from the R/V VALDIVIA, during cruise VAL-175, which was used for teaching purposes at that time. Air-gun shooting as well as two landshots were used for seismic source, while recording was done at sea as well as on land

Heeremans, Michael; Faleide, Jan Inge & Larsen, Bjorn Tore (2000). Permo-Carboniferous rifting and magmatism in the Skagerrak, Kattegat and the North Sea: evidence from seismic, borehole and potential field data.
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In the present study we focus on Permo-Carboniferous rifting and related magmatic activity in northwestern Europe. We are especially interested in the distribution of intrusives and extrusives in relation to rift geometries. A large database containing seismic and borehole data has been put together and analysed to constrain these objectives. The continuation of the Oslo Rift into Skagerrak has been a starting point for this regional study of Permo-Carboniferous rifting and magmatism. Permo-Carboniferous rift structures (with characteristic half graben geometries) and the distribution of magmatic rocks (intrusives and extrusives) have been mapped using integrated analyses of seismic, gravity and magnetic data. A similar approach has been used in the Kattegat and the North Sea, where the interpretation is calibrated with well data. The Permo-Carboniferous rift structures in Skagerrak can be followed into the Sorgenfrei-Tornquist zone where similar fault geometries have been observed. Both in Skagerrak and Kattegat, thick lava sequences have been deposited which are mainly parallel with the underlying Palaeozoic strata. This volcanic episode therefore predates the main fault movements, which gave rise to the half graben geometries filled with Permian volcano-clastic material. Permian extrusives and intrusives have also been found in wells in Kattegat, Jutland and in the North Sea (Horn and Central Grabens). Especially in the last area, the dense seismic and well coverage has allowed us to map out similar late-Palaeozoic rift geometries, although the presence of salt often conceals the seismic image of the underlying strata and structures. Our results show that the pre-Jurassic structures below large parts of the Norwegian-Danish Basin and northwards into the Stord Basin on the Horda Platform most likely belong to the same rift system, where the apparent age of faulting is late-Carboniferous(?)-early Permian.

Heeremans, Michel (1997). Silicic Magmatism and Continental Lithosphere Thinning: Inferences from field studies and numerical modelling of the Oslo Graben and the anorogenic crustal evolution of southern Finland.